Silica is commonly used as stationary phase for chromatography, e.g. High Performance Liquid Chromatography (HPLC). Unlike organic stationary phases, chromatographic silica materials generally provide an improved efficiency and show less evidence of swelling or shrinking. Typically, chromatographic applications employ silica which has been functionalized with an organic group such as octadecyl, octyl, phenyl, amino, cyano and the like.
A disadvantage of silica materials is the presence of residual silanol groups, which may interact with analytes. This may result in excessive peak tailing and irreversible adsorption of some analytes. A further drawback of silica materials is their limited hydrolytic stability. Indeed, the incomplete derivatization of the silica leaves patches of bare silica surface which can be dissolved under alkaline conditions. Furthermore, the silica materials may lose their functional groups under acidic or basic conditions, causing loss of analyte retention and an increase in the concentration of surface silanol groups.
Various solutions to overcome these problems have been developed.
U.S. Pat. No. 7,563,367 (Phenomenex Inc.) describes pH stable chromatographic media using templated multilayer organicinorganic grafting. Although the described compounds exhibit a wider pH range stability as compared to other silica gel sorbents, the functional groups are still easily lost. Furthermore, the preparation of the described compounds requires an extra post synthesis modification step which is not completely reproducible and results in a loss of separation efficiency.
U.S. Pat. No. 7,919,177 (Waters Technologies Corporation) describes porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry, which may be surface modified. However, the particles still are unstable at increased pH.
U.S. Pat. No. 7,947,799 (Landskron et al.) describes silica based mesoporous films having an increased number of bridging organic groups. Although these materials have an increased mechanical stability, the short bridging organic groups have a limited influence on separation efficiency.
Accordingly, there is still a need for improved stationary phases for chromatography. More particular, there is a need for silica material to which a functional group is or can be bound which cannot be removed by an acidic or alkaline solution.